RAINWATER RETENTION SITE ASSESSMENT FOR URBAN FLOOD RISK REDUCTION AND FLOOD DEFENCE IN MANDAUE CITY, PHILIPPINES

Abstract. This study utilized a geospatial approach to identify suitable sites for rainwater retention areas in Mandaue City, a highly urbanized city in Cebu, Philippines. Based on the integrated use of remote sensing and Geographic Information System (GIS), ideal sites for rainwater retention were produced to divert surface water runoff from flooding the streets and obstructing traffic. Ideally, the chosen sites should have low infiltration capacity and be within open spaces. For the hydrological planning of the study, we considered the amount of surface runoff, elevation, soil characteristics, land use, and present drainage status of the study area. The Digital Elevation Model (DEM) derived from LiDAR technology is used to obtain the slope raster and generate small streams. Land cover and curve number (CN) of the area were used to compute the surface runoff. The soil textures were converted based on their infiltration rate and runoff potential. Finally, the weighted overlay tool in GIS created a layer identifying potential areas for rainwater retention sites. The findings indicate that 54.8% of the research area was ideal for water harvesting, with open land areas making up 2.3% of the best locations. In addition, a major portion of the appropriate locations lies in clay loam soils and within 0–25 meters in elevation. These results will hugely benefit policymakers and urban planners in helping create solutions to the city’s flooding problem and the looming water crisis.